This invention relates to the field of automotive hydraulic systems and has particular application to automotive hydraulic systems having a power steering system and another hydraulically powered device connected in series with the power steering system. In a typical application the other device may be a cooling fan run by a hydraulic motor.
Hydraulic fluid for a power steering unit is generally delivered by a constant flow rate pump. Flow continues at the prescribed volumetric rate, irrespective of system back pressure, so long as the pump is able to deliver it. That necessarily involves a risk of pump damage. Therefore pumps for such systems generally are provided with pressure relief lines which terminate the pumping action in case of excessive system loads. This saves the pump at the expense of temporary impairment of power steering and temporary loss of service from anything else which may be powered by the pump. Sometimes bypass lines are provided around individual components of the system, so as to avoid loss of the entire system when a localized abnormality is experienced.
Proper pump design calls for a pump having only the ability to supply the normal needs of the devices which it is serving. If a plurality of devices are being served, it is desirable that they share the hydraulic fluid on some basis of priority. Clearly power steering is a matter of high priority. So long as a power steering unit is operating normally, its needs for hydraulic fluid should receive preferred treatment.
A cooling fan performs an essential function in protecting the automotive engine from overheating. However, the fan operation may be temporarily halted without serious risk to the motor vehicle or to its passengers. This means that a cooling fan should operate in series with a power steering unit only on a low priority basis. If the load on the pump becomes excessive, then the needs of the cooling fan should be sacrificed.